Try this:
Consider a two-dimensional gas of molecules inside a rectangular box of small width d and very large length. If the box is filled by point-like molecules with density n, then the pressure is p=nkBT. (kB is the Boltzmann constant, and T is the temperature.) If the point-like molecules are replaced by thin linear molecules of length L, then the pressure will slightly increase.
(a) Consider a case when L is smaller (but not much smaller) than d, and the molecules do not interact with each other, but they cannot penetrate the walls of the box. (See picture (a).) Calculate the exact pressure in the box.
This one was on one of my tests,
Is it possible that two different transitions in hydrogen atom give the same frequency of radiation?
-Kastro